Processes for the conversion of biomass to oxygenated organic compound, apparatus therefor and compositions produced thereby

ABSTRACT

Processes are disclosed for the conversion of biomass to oxygenated organic compound using a simplified syngas cleanup operation that is cost effective and protects the fermentation operation. The processes of this invention treat the crude syngas from the gasifier by non-catalytic partial oxidation. The partial oxidation reduces the hydrocarbon content of the syngas such as methane, ethylene and acetylene to provide advantageous gas feeds for anaerobic fermentations to produce oxygenated organic compounds such as ethanol, propanol and butanol. Additionally, the partial oxidation facilitates any additional cleanup of the syngas as may be required for the anaerobic fermentation. Producer gases and partial oxidation processes are also disclosed.

FIELD OF THE INVENTION

This invention pertains to processes and apparatus for convertingbiomass to oxygenated organic compound in a commercially attractivemanner and to producer gas compositions.

BACKGROUND

Numerous proposals exist for the gasification of biomass to producegases containing carbon monoxide, hydrogen and carbon dioxide. Forpurposes herein, the gases from gasification operations are referred toas synthesis gas (syngas). Anaerobic fermentations of carbon monoxideand hydrogen and carbon dioxide have also been proposed and involve thecontact of the substrate gas in a liquid, aqueous menstruum withmicroorganisms capable of generating oxygenated organic compounds suchas ethanol, acetic acid, propanol and n-butanol. The production of theseoxygenated organic compounds requires significant amounts of hydrogenand carbon monoxide. For instance, the theoretical equations for theconversion of carbon monoxide and hydrogen to ethanol are:6CO+3H₂O.C₂H₅OH+4CO₂6H₂+2CO₂.C₂H₅OH+3H₂O.

As can be seen, the conversion of carbon monoxide results in thegeneration of carbon dioxide. The conversion of hydrogen involves theconsumption of hydrogen and carbon dioxide, and this conversion issometimes referred to as the H₂/CO₂ conversion. For purposes herein, itis referred to as the hydrogen conversion.

The microorganisms for the anaerobic fermentation of syngas can beadversely affected by components contained in syngas. See, for instance,Xu, et al., The Effects of Syngas Impurities on Syngas Fermentation toLiquid Fuels, Biomass and Bioenergy, 35 (2011), 2690-2696; United StatesPublished Patent Application No. 20110097701; Abubackar, et al.,Biological Conversion of Carbon Monoxide: Rich Syngas or Waste Gases toBioethanol, Biofuels, Bioproducts & Biorefining, 5, (2011), 93-114; andMunasinghe, et al., Biomass-derived Syngas Fermentation into Biofuels:Opportunities and Challenges, Bioresource Technology, 101, (2011),5013-5022.

Numerous processes have been suggested for the cleanup of syngas foranaerobic fermentation. Often, the processes involve multiple operationsto remove different adverse components from the syngas. Xu, et al.,state at page 2692:

-   -   “Syngas impurities may or may not need to be removed depending        upon the effect of the impurity on the biological process and        the environment. Selection of commercial technologies suitable        for syngas cleanup is mainly based on affordability and the        ability to meet end user specifications . . . . Currently, tar        cracking methods (including cracking within the gasifier) can        effectively convert the heavy and light hydrocarbons to        negligible levels. Water quench scrubbers can be employed for        removal of ammonia and trace impurities. Accordingly, amine        treatment can be utilized for sulfur and CO₂ treatment after        cooling down the syngas. Zinc oxide beds can also be added for        additional sulfur removal down to low levels meeting the        requirement for fuel synthesis . . . . For fermentation        processes using CO₂ as one of the substrates, a different sulfur        treatment method should be considered. Alternatively, H₂S can be        removed from the gasification processes by using regenerable        mixed oxide sorbents such as Zinc titanates . . . . ”    -   “Hot catalytic gas conditioning downstream of the gasifier        demonstrates more advantages than physical strategies        (scrubber+filter). Catalytic strategies provide the possibility        to transform the impurities (especially tars and ammonia) into        useful gas compounds. By adding cobalt and nickel promoters to        Zn—Ti sorbents, both NH₃ decomposition and H₂S adsorption will        occur simultaneously. Most literature has centered on converting        tars into useful gases on basic (calcined dolomites) and        alumina-supported nickel catalysts at temperatures between 973        and 1173 K. The coupling of a guard bed made from calcined        dolomite with a nickel catalytic unit can effectively reduce tar        levels to a few ppms . . . . ”

Syngas is typically more expensive than equivalent heat content amountsof fossil fuels. Hence, a desire exists to use syngas efficiently bothin the fermentation operation to make higher value products and inconserving the syngas values in any cleanup operation. The financialviability of any conversion process, especially to commodity chemicalssuch as ethanol and acetic acid, will be dependent upon capital costs aswell as the efficiency of conversion of the carbon monoxide and hydrogento the sought products and the energy costs to effect the conversion.

The cleanup of syngas from biomass is further complicated since biomassis subject to variabilities that can affect gasifier performance andsyngas composition. Moreover, a gasifier may from time to time changetypes of biomass being gasified which may also result in changes ingasifier performance and syngas composition. Thus variabilities inconcentrations of components adverse to the fermentation operation, suchas hydrogen cyanide, nitric oxide, acetylene and ethylene, occur.Consequently, any cleanup operation would need to have sufficientcapacity to handle peak amounts of impurities. Also, the cleanupoperation would have to have sufficient turndown capabilities as may berequired for cleaner syngas from the gasifier and for startup andnon-steady-state operations.

For a biomass to oxygenated organic compound fermentation process to becommercially viable, capital and operating costs must be sufficientlylow that it is at least competitive with alternative biomass tooxygenated organic compound processes. For instance, ethanol iscurrently commercially produced from corn and cane sugar in facilitieshaving name plate capacities of over 100 million gallons per year atsufficiently low costs to be competitive with fossil fuels. Biomass tooxygenated organic compound fermentation processes face even greaterchallenges due to the multiple major operations required to convert thebiomass to syngas, cleanup the syngas sufficiently to be used in ananaerobic fermentation, effect the anaerobic fermentation and thenrecover a merchantable product.

United States Published Patent Application No. 20100237290 discloses amethod for producing a purified syngas from the severe pyrolysis ofbiomass comprising removing dust and sulfur compounds from the pyrolysisgas, then subjecting the pyrolysis gas to partial oxidation at pressuressuitable for conducting a Fischer-Tropsch synthesis and rapidly coolingthe gas to a temperature of between 300° C. and 500° C. The patentapplicants state that their purified synthesis gas can be used as afeedstock of a Fischer-Tropsch synthesis unit for making liquid fuelsand for the synthesis of ammonia, alcohols or dimethyl ether. The patentapplicants do not disclose or suggest the use of their purified syngasfor anaerobic fermentation nor do they provide any indication of thecontent in the purified syngas of components that can adversely affectfermentation. Indeed, such components such as benzene and higheraromatics, ethylene and other alkenes, and acetylene and other alkyleneswould be desirable in a feedstream to a Fischer-Tropsch synthesis unit.

Processes are sought to convert biomass to oxygenated organic compoundat low capital and operating cost but yet provide sufficient robustnessthat variations in biomass feedstock and gasifier performance can occurwithout adversely affecting syngas fermentation. Accordingly, theprocesses need to be characterized by cost-effective syngas cleanup withminimal loss of carbon monoxide and hydrogen yet be able to protect thefermentation from adverse components despite changes in biomassfeedstock and changes in gasifier performance.

SUMMARY OF THE INVENTION

In accordance with this invention processes are provided for theconversion of biomass to oxygenated organic compound using a simplifiedsyngas cleanup operation that is cost effective and protects thefermentation operation. The processes of this invention treat the crudesyngas from the gasifier by non-catalytic partial oxidation. The partialoxidation provides several advantages beyond reducing the hydrocarboncontent such as methane, ethylene and acetylene contained in the crudesyngas. Namely, the partial oxidation increases the concentration ofboth hydrogen and carbon monoxide and lowers the hydrogen to carbonmonoxide mole ratio which is particularly advantageous in producingsyngas for anaerobic fermentation. The partial oxidation generallyresults in a reverse water gas shift being observed such that the carbondioxide concentration is decreased. Accordingly, a higher percentage ofthe biomass becomes available for bioconversion to oxygenated organiccompound.

Also, the partial oxidation materially reduces the concentration ofother components that may adversely affect the fermentation. Thesecomponents include nitric oxide, nitrogen dioxide, and hydrogen cyanide.The reduction of the hydrocarbons and other components may be sufficientsuch that little, if any further clean-up treatment is required. Even iffurther clean-up treatment is required, the concentration of componentssuch as alkenes, alkynes and light aromatics such as benzene, toluene,xylene and naphthalene along with heavy tars, is so reduced by thepartial oxidation that the process operation need only remove a smallamount of these components. Importantly, the concentrations of thesecomponents generally fall within a small range even though theirconcentrations in the crude syngas may vary widely due to variations ingasifier performance and in biomass.

The partial oxidation in accordance with the processes of this inventionis conducted without the need for sulfur compound removal operations inadvance of the partial oxidation, e.g., for the removal of hydrogensulfide, carbonyl sulfide and organosulfur compounds such as mercaptans.

The broad aspect of the processes of this invention for continuouslyconverting biomass into oxygenated organic compound comprise:

-   -   (a) continuously gasifying biomass at elevated temperature to        provide a crude syngas at a temperature of at least about 450°        C., said crude syngas having a Component Composition containing        carbon monoxide, hydrogen, and carbon dioxide and at least about        3, sometimes at least about 5, say, 5 to 15, mole percent        methane; at least about 100 ppm (mole) hydrogen sulfide, at        least about 600, sometimes at least about 1000, say 1000 to        7000, ppm (mole) benzene, at least about 1000, sometimes at        least about 2000, say, 2500 to 12,000, ppm (mole) ethylene, at        least about 500 ppm (mole) acetylene, at least about 20, often        at least about 25 to 50, ppm (mole) hydrogen cyanide, and ash;    -   (b) removing by phase separation a major portion by mass of the        ash while the crude syngas is maintained at a temperature of at        least about 350° C. to provide an ash-reduced crude syngas;    -   (c) continuously contacting the ash-reduced crude syngas with        oxygen-containing gas under partial oxidation conditions to        perform a partial oxidation at a temperature of between about        800° C. to 1700° C., preferably 1150° C. to 1500° C., and for a        time sufficient to provide a partially-oxidized syngas having a        Component Composition containing less than about 1, preferably        between about 0.1 and 0.75, and often between about 0.1 and 0.4,        mole percent methane and at least the same molar concentration        of carbon monoxide as contained in the ash-reduced crude syngas        on a Component Composition basis, said partially-oxidized syngas        having a Component Composition containing at least about 80 ppm        (mole) hydrogen sulfide, less than 200 ppm (mole) ethylene, less        than 100 ppm (mole) acetylene;    -   (d) continuously cooling the partially-oxidized syngas at least        partially by contact with water to cool the partially-oxidized        syngas to a temperature below about 100° C., preferably below        about 50° C., and provide a quenched syngas containing less than        about 80 ppm (mole) hydrogen sulfide on a Component Composition        basis; and    -   (e) removing hydrogen cyanide from the quenched syngas by at        least one of sorption and chemical reaction to provide a        fermentation gas feed with a Component Composition having a        hydrogen cyanide content of less than about 2, preferably less        than about 0.5, ppm (mole); and    -   (f) continuously supplying the gas feed to a fermentation zone        having an aqueous menstruum containing microorganisms suitable        for converting syngas to oxygenated organic compound, said        aqueous menstruum being maintained under anaerobic fermentation        conditions, to produce said oxygenated organic compound.

The gasification of step (a) may be a direct, indirect or partiallydirect gasification. An indirect gasification does not use free oxygen,but an external source of heat is required. Direct gasification occurswhere a oxygen is used to partially oxidize the biomass. Where thegasifying of step (a) is conducted in a direct or partially direct mode,the crude syngas often has a Component Composition containing at leastabout 50 ppm (mole) nitric oxide. Where the gasifying of step (a) isconducted in an indirect mode, i.e., heat for the gasification isindirectly supplied, the nitric oxide content of the ComponentComposition of the crude syngas is typically less than about 5 ppm(mole). In the preferred processes of this invention, the gasificationis conducted by supplying the heat for gasification at least partiallyin an indirect mode.

Preferably the partial oxidation of the crude syngas in step (c) isnon-catalytic. The oxygen-containing gas for the partial oxidation maybe air or air enriched with oxygen. Where the partial oxidation isconducted using oxygen enriched air or oxygen, the oxygen-containing gashas a Component Composition having an oxygen content of at least about50, preferably at least about 90, and most preferably at least about 98,mole percent oxygen. The presence of nitrogen in the oxygen-containinggas can led to the production of nitric oxide. Where air is used as theoxygen-containing gas, the nitric oxide concentration of the ComponentComposition of the treated syngas may be as high as 1000 ppm (mole).With oxygen-enriched oxygen-containing gas, the nitric oxideconcentration in the partially oxidized syngas Component Composition maybe in the range of 0.1 to 100 ppm (mole). The partial oxidation may beconducted by at least one of directly contacting the crude syngas withoxygen-containing gas and by admixing a fuel with oxygen-containing gasprior to contacting with the crude syngas. In the latter case,preferably the fuel is partially combusted to provide heat and anoxygen-containing mixture. The heat raises the temperature of the fueland oxygen-containing mixture to desired temperatures for the partialoxidation and may also be used in indirect heat exchange with the crudesyngas to increase its temperature.

A further aspect of this invention is a continuous process for thepartial combustion of crude syngas having a Component Compositioncontaining at least about 3 mole percent methane comprising:

-   -   (a) continuously contacting a stream containing crude syngas        with an oxygen-containing stream under partial oxidation        conditions including a temperature of between about 800° C. to        1700° C., preferably 1150° C. and 1500° C., and a rate of supply        of said oxygen-containing stream sufficient to perform a partial        oxidation to provide a partially-oxidized syngas;    -   (b) determining the methane content in the partially-oxidized        syngas; and    -   (c) adjusting the rate of supply of the oxygen-containing stream        such that the partial oxidation conditions provide a        partially-oxidized syngas having a Component Composition        containing between about 0.1 and 0.75, preferably between about        0.1 and 0.4, mole percent methane.        In a preferred embodiment of this aspect of the invention, the        oxygen-containing stream is a mixture of hydrocarbon fuel and        oxygen-containing gas and the mole ratio of hydrocarbon fuel and        oxygen-containing gas is also adjusted to provide a        partially-oxidized syngas having a Component Composition        containing between about 0.1 and 0.75, preferably between about        0.1 and 0.4, mole percent methane. In some instances, the        hydrocarbon fuel and oxygen-containing gas is partially        combusted prior to contacting the stream containing crude        syngas. The partial combustion provides heat and an        oxygen-containing mixture. The heat raises the temperature of        the oxygen-containing mixture to desired temperatures for the        partial oxidation and may also be used in indirect heat exchange        with the crude syngas to increase its temperature.

A further aspect of this invention pertains to partially-oxidized syngascompositions. The processes of this invention provide compositions uponpartial oxidation of the crude syngas having a Component Compositioncomprising:

-   -   (a) hydrogen and carbon monoxide wherein the mole ratio of        hydrogen to carbon monoxide is between about 0.4:1 to 1.5:1,        preferably between about 0.8:1 and 1.3:1, and wherein hydrogen        and carbon monoxide comprise at least about 70, preferably at        least about 75, and most preferably at least about 80, mole        percent of the syngas composition;    -   (b) between about 0.1 and 0.75, preferably between about 0.1 and        0.4, mole percent methane;    -   (c) between about 1 and 100, preferably between about 1 and 30,        ppm (mole) acetylene;    -   (d) between about 10 and 200, preferably between about 10 and        70, ppm (mole) ethylene;    -   (e) between about 0.1 and 50, say, 0.5 to 20, ppm (mole)        hydrogen cyanide; and    -   (f) between about 2 and 25, preferably between about 5 and 15,        mole percent carbon dioxide.        The syngas will contain water and nitrogen. The amount of        nitrogen will, in part, depend upon the composition of the        feedstock, nature of the gasification process, and the        oxygen-containing gas used for the partial oxidation. Where an        indirect gasification process is used and partial oxidation is        conducted using an oxygen-containing gas having a high molecular        oxygen content, the nitrogen content can be relative low, say,        from about 0.1 to 5 mole percent of the Component Composition of        the syngas. Direct gasification processes using air and partial        oxidation processes using air, can yield a syngas Component        Composition with nitrogen concentrations as high as 55 to 60        mole percent of the syngas. The preferred syngas Component        Compositions contain at least about 20, often between about 80        and 300, ppm (mole) hydrogen sulfide and contain between about 2        and 100, more frequently between about 3 and 50, say, 3 to 30,        ppm (mole) benzene. The syngas Component Composition usually        contains other components such as tars such as naphthalene and        heavier aromatics (generally less than about 500, preferably        less than about 150, ppm (mole)), lighter aromatics other than        benzene such as toluene and xylene (often between about 2 and 70        ppm (mole)), carbonyl sulfide (often between 0.1 and 25 ppm        (mole)), and ammonia (often between about 100 and 10,000 ppm        (mole)).

Another aspect of this invention pertains to apparatus for theconversion of biomass to oxygenated product comprising:

-   -   (a) at least one gasifier capable of converting biomass to        syngas at a temperature of at least about 450° C. having at        least one biomass input port and at least one syngas exit port;    -   (b) at least one solids removal device capable of operating at        least at 350° C. in fluid communication with at least one syngas        exit port of a gasifier and capable of removing solids from        syngas;    -   (c) at least one partial oxidation unit having at least one        crude syngas inlet port, at least one oxygen inlet port and at        least one partially-oxidized syngas exit port, said crude syngas        inlet port being in fluid communication with at least one solids        removal device, and said at least one oxygen inlet port being in        fluid communication with a source of an oxygen-containing gas;    -   (d) at least one methane detector in fluid communication with at        least one partially-oxidized syngas exit port capable of        determining the concentration of methane in the gas passing        through the at least one partially-oxidized syngas exit port and        generating a control signal corresponding to such concentration;    -   (e) a controllable valve positioned between the source of an        oxygen-containing gas and the at least one oxygen inlet port        that is controlled in response to the control signal from the at        least one methane detector;    -   (f) at least one heat exchanger in fluid communication with at        least one partially-oxidized syngas exit port adapted to contact        gas from the at least one partial oxidation unit with aqueous        medium to provide a cooled gas at a temperature less than about        100° C., preferably less than about 50° C., at a cooled gas exit        port, at least one of said heat exchangers adapted to provide        direct contact of the partially-oxidized syngas with water;    -   (g) at least one hydrogen cyanide removal unit in fluid        communication with the cooled gas exit port adapted to provide        at one or more treated exit ports a hydrogen cyanide depleted        gas;    -   (h) at least one fermentor in fluid communication with the at        least one treated exit port, said fermentor containing an        aqueous menstruum under anaerobic fermentation conditions, said        aqueous menstruum comprising microorganisms suitable for        converting carbon monoxide and hydrogen and carbon dioxide to        oxygenated organic compound, said fermentor comprising at least        one gas exit port and at least one liquid exit port; and    -   (i) at least one oxygenated organic compound recovery unit in        fluid communication with the at least one liquid exit port        adapted to recover oxygenated organic compound from said aqueous        menstruum.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of apparatus suitable to carry outthe processes of this invention.

DETAILED DISCUSSION Definitions

The term Component Composition means the composition of a gas where bothwater and nitrogen have been excluded from the calculation of theconcentration of the components. As used herein, unless otherwisestated, compositions of gases are on an anhydrous basis and exclude thepresence of nitrogen.

Oxygenated organic compound means one or more organic compoundscontaining two to six carbon atoms selected from the group of aliphaticcarboxylic acids and salts, alkanols and alkoxide salts, and aldehydes.Often oxygenated organic compound is a mixture of organic compoundsproduced by the microorganisms contained in the aqueous menstruum.

Aqueous menstruum means a liquid water phase which may contain dissolvedcompounds including, but not limited to hydrogen, carbon monoxide, andcarbon dioxide.

Biomass means biological material living or recently living plants andanimals and contains at least hydrogen, oxygen and carbon. Biomasstypically also contains nitrogen, phosphorus, sulfur, sodium andpotassium. The chemical composition of biomass can vary from source tosource and even within a source. Sources of biomass include, but are notlimited to, harvested plants such as wood, grass clippings and yardwaste, switchgrass, corn (including corn stover), hemp, sorghum,sugarcane (including bagas), and the like; and waste such as garbage andmunicipal waste. Biomass does not include fossil fuels such as coal,natural gas, and petroleum.

The abbreviation ppm means parts per million. Unless otherwise stated orclear from the context, ppm is on a mole basis (ppm (mole)).

Stable gas-in-liquid dispersion means a mixture of gas bubbles in liquidwhere (i) the bubbles predominantly flow in the same direction as theliquid, and (ii) the dispersion is sufficiently stable that it existsthroughout the aqueous menstruum, i.e., insufficient coalescing ofbubbles occurs to destroy the dispersion.

For a syngas to oxygenated organic compound fermentation process to becommercially viable, capital and operating costs must be sufficientlylow that it is at least competitive with alternative biomass tooxygenated organic compound processes. For instance, ethanol iscommercially produced from corn in facilities having name platecapacities of over 100 million gallons per year. Accordingly, the syngasto oxygenated organic compound fermentation process must be able to takeadvantage of similar economies of scale. Thus, a commercial scalefacility may require at least 20 million liters of fermentation reactorcapacity.

Overview

The processes of this invention pertain to the conversion of biomass tooxygenated organic compound by gasification to provide a substratecontaining carbon monoxide, hydrogen and carbon dioxide andbioconversion of the substrate to the oxygenated organic compound viaanaerobic fermentation.

Biomass Gasification

Gasification is a thermal process to convert biomass. The gasificationmay be effected by any suitable process to provide a gas containingcarbon monoxide and hydrogen and may involve the presence of controlledamounts of oxygen and steam. Typically gasification involves heating thebiomass in an oxygen-controlled environment. The heat may be provided bydirect or indirect heat exchange as stated above. Various types ofgasifiers include pyrolysis, counter current fixed bed, co-current fixedbed, moving bed, fluidized bed, entrained flow and plasma gasifiers. Onetype of gasification process is the Taylor gasification processgenerally disclosed in United States published patent application No.2008024496 A1, hereby incorporated by reference in its entirety. Theprocess may involve a combination of pyrolysis and steam reforming. Inaccordance with this invention, gasification occurs at a temperature ofat least about 450° C., often between about 500° C. and 1500° C., say,600° C. to 1250° C. The gasification may be conducted at any suitablepressure including subatmospheric pressure, but is typically conductedat pressures from about 100 to 5000 KPa absolute.

Typically the biomass is pre-conditioned prior to gasification, forinstance by communition to provide biomass of suitable size for thegasification process used and by drying to provide a moisture content ofless than about 30 mass percent, often in the range of about 5 to 20mass percent. Maintaining a relatively constant moisture content of thebiomass facilitates control of the gasification process. Usually energyfrom the conversion process is recovered to provide heat for the dryingin order to enhance overall efficiency of the process to make oxygenatedorganic compound. Where biomass to be fed has high moisture content,such as municipal waste, admixture with another, drier biomass mayfacilitate handling and reduce the amount of energy required to achievethe sought moisture content for the biomass feed. If desired, thebiomass being fed to the gasifier can be contacted with the crude gasstream from the gasifier to heat, and in some instances, initiatepyrolysis of at least a portion of the biomass feed. Advantageously, ifsuch a step is used, the crude syngas is not cooled below about 350° C.

Gasification of biomass, however, leads to the formation of a complexset of by-products due to the nature of the biomass constituents and theconditions of the gasification. These by-products include tars, char andash, light hydrocarbons, nitrogen oxides, other nitrogenous compoundssuch as hydrogen cyanide and sulfur compounds. To some extent, theformation of by-products can be affected by the conditions of thegasification as is known in the art. Lower gasification temperaturesgenerally tend to increase the content of tars and lighter hydrocarbons.Higher gasification temperatures generally tend to increase operatingand capital costs. Table I provides typical crude syngas ComponentCompositions that are produced by some available gasification unitoperations.

TABLE I Preferred Preferred Component Minimum Maximum Minimum MaximumCarbon Monoxide, 20 60 30 60 mole % Hydrogen, mole % 20 60 30 60 CarbonDioxide, mole % 5 35 10 25 Methane, mole % 3 15 3 12 Acetylene,ppm(mole) 40 2500 200 2000 Ethylene, ppm(mole) 400 25000 1000 12000Benzene, ppm(mole) 600 10000 1000 7500 Tars, naphthalene, 100 10000 2005000 ppm(mole) Hydrogen sulfide, 80 1000 80 500 ppm(mole) Carbonylsulfide, 3 100 5 50 ppm(mole) Ammonia, ppm(mole) 100 10000 500 7500Nitric oxide, ppm(mole) 0.5 1000 1 500 Hydrogen cyanide, 20 100 20 50ppm(mole) Other, ppm(mole) 100 100000 200 20000 (Not including water andnitrogen)

Solids Removal

Gasification of biomass results in the formation of ash (includingchar). The amount of these solids present in the crude syngas willdepend not only upon the type of gasifier used but also the nature ofthe biomass. In accordance with the processes of this invention, thecrude syngas from the gasifier is maintained at a temperature of atleast about 350° C., preferably at least about 500° C. or 600° C., andsometimes in the range of about 700° C. to 1500° C. This providesseveral advantages. First, equipment required to further cool the crudesyngas is not required resulting in a capital cost savings. Second, asthe gasification process is conducted at an elevated temperature, heatrequired to increase the temperature of the crude syngas is reduced.Third, higher boiling hydrocarbon containing components that are capableof being partially-oxidized to form carbon monoxide and hydrogen aremaintained in the gas phase. And fourth, maintaining the crude syngas atthese high temperatures reduces the formation of waxy deposits on thepiping and equipment.

However, gasification of biomass results in the formation of ash(including char) and at least some of the ash will be entrained in thecrude syngas. The amount of these solids present in the crude syngaswill depend not only upon the type of gasifier used but also the natureof the biomass. The processes of this invention comprise removing atleast about 75, preferably at least about 90 to essentially 100, masspercent of the solids from the crude syngas. The removal of solids canbe effected in any suitable manner. Cyclones are preferred since in mostinstances, cyclones are capable of removing sufficient entrained solids.

The solids removed from the crude syngas and any solids removed from thegasifier usually contain non-volatilized tars. If desired, additionalheat and syngas values can be recovered by subjecting these solids tooxidation conditions preferably including temperatures of at least about700° C. and the presence of oxygen-containing gas such as air especiallywhere heating values are sought, or oxygen or oxygen enriched air whereadditional syngas is sought to be produced without undue nitrogencontent.

Partial Oxidation

The partial oxidation step of this invention benefits from having acrude syngas feed that is already at an elevated temperature, i.e., at atemperature of at least about 350° C. The partial oxidation may beconducted in any suitable manner to effect a reduction in methanecontent of the syngas Component Composition to less than about 1,preferably between about 0.1 and 0.75, often between about 0.1 and 0.4,mole percent. As stated above, other components of the syngas such asother hydrocarbons, nitrogen oxides, and hydrogen cyanide, are alsoaffected by the partial oxidation conditions that provide the reductionof methane content. Accordingly, a partially-oxidized syngas is producedthat may require little, if any, further cleanup treatment to be asatisfactory gas feed for anaerobic fermentation to produce oxygenatedorganic compound.

Since the crude syngas contains sulfur compounds, the partial oxidationis preferably conducted in a non-catalytic manner. The partial oxidationis typically conducted at a temperature in the range of between about800° C. to 1700° C., say, 1150° C. and 1500° C., preferably about 1250°C. and 1500° C. Generally the partial oxidation is substantiallyadiabatic, and thus the targeted temperature will be achieved by acombination of the temperatures, duration of the partial oxidation andrelative mass flow rates of the input gases and the heat generated bythe partial oxidation. All else being substantially the same, lowerpartial oxidation temperatures tend to result in higher methaneconcentrations in the partially-oxidized syngas and higher partialoxidation temperatures tend to result in higher carbon dioxideconcentrations in the partially-oxidized syngas. While not intending tobe limited to theory, it is believed that the generation of freeradicals play a role in the partial oxidation of the crude syngas. Wherethe crude syngas contains free radical inhibitors such as halides, itmay be desirable to use higher temperatures for the partial oxidation.The partial oxidation may be conducted at subatmospheric, atmospheric orsuperatmospheric pressure. Typically the pressure used for the partialoxidation is in the range of between about 100 and 3000, preferablybetween about 100 and 500, KPa absolute. The conditions of the partialoxidation are preferably such that combustion is avoided or mitigated.Often the velocity of the total gases during partial oxidation issufficiently high to avoid a flame front, for instance, at least about150, preferably at least about 200, say, between about 250 and 500,meters per second. At these partial oxidation conditions, the partialoxidation can be conducted relatively rapidly.

Oxygen-containing gas is admixed with the crude syngas for purposes ofthe partial oxidation. The source of the oxygen may be air,oxygen-enriched air or substantially pure oxygen. Where it is desired toavoid undue nitrogen dilution of the syngas, the source of the oxygenfor the oxygen-containing gas preferably contains at least about 75,preferably at least about 90, and most preferably at least about 98,mole percent oxygen (concentration including the presence of nitrogenbut is on an anhydrous basis). The oxygen-containing gas prior tocontacting the crude syngas may contain components other than oxygen andnitrogen, including, but not limited to, hydrogen, carbon monoxide,carbon dioxide, hydrocarbon-containing compounds (preferablyhydrocarbon-containing compounds having between about 1 and 20 carbons),and water vapor. The amount of oxygen provided is sufficient to providethe sought partial oxidation temperature and reduction of methane in thetreated syngas. The amount of oxygen required for a specific crudesyngas composition will depend upon the composition of the syngas,whether or not any additional fuel is provided for the partialoxidation, the concentration of methane and additional fuel in the crudesyngas composition and the extent the methane and any additional fuel isdesired to be converted into incrementally additional syngas. Ingeneral, the mass ratio of oxygen to total methane and any addedhydrocarbon fuel is between about 0.3:1 to 3.5:1.

As stated above, one convenient method for effecting the partialoxidation of the crude syngas with conservation of carbon monoxide andhydrogen values comprises adding a hydrocarbon-containing fuel to thecrude syngas. The terms “hydrocarbon-containing fuel” and “fuel” as usedherein refer to fuels that contain hydrogen and oxygen atoms and maycontain hetero atoms including, but not limited to oxygen and nitrogenatoms. Typically hydrocarbon-containing fuels are less expensive thansyngas. Combustion of the fuel provides heat to provide the soughtpartial oxidation temperature. Hydrocarbon-containing fuels include, butare not limited to, natural gas, propane, liquified petroleum gas,butane, fuel oil, petroleum fractions having normal boiling pointsbetween about 35° C. and 350° C., and oxygenated hydrocarbons such asalkanols (such as methanol, ethanol, propanol, and butanol) diols (suchas ethylene glycol), esters, ethers, and carboxylic acids of 1 to 20carbons. The amount of fuel added will, in part, depend upon the amountof heat necessary to be generated to achieve the sought partialoxidation temperature. More hydrocarbon-containing fuel can be added,and its partial oxidation will result in the production of additionalsyngas. Often, the amount of hydrocarbon-containing fuel added is atleast about 0.5, say, at least about 1, and sometimes between about 1and 10, for instance, 1 and 5, mass percent of the crude syngas on aComponent Composition basis.

The fuel may be added before combustion to the crude syngas, or may bepartially or substantially fully combusted when added to the syngas toeffect the partial oxidation of the crude syngas. Preferably the fuel isin admixture (fuel/oxygen admixture) with oxygen before combination withthe crude syngas, that is, the fuel and oxygen from the source of oxygenis the oxygen-containing gas. In some instances, a portion of thefuel/oxygen admixture is partially combusted prior to admixture with thesyngas (pre-reaction). Frequently, the pre-reaction of the fuel/oxygenadmixture consumes from about 80 or 90 to essentially 100 mass percentof the fuel. The pre-reaction may be under the same or differentconditions than those for the partial oxidation of the syngas. Thepre-reaction may be catalytic or non-catalytic. The pre-reaction servesto increase the temperature of the fuel/oxygen admixture and can be usedto increase the temperature of the crude syngas by direct or indirectheat exchange. Examples of types of apparatus for providing andpre-reacting the fuel/oxygen admixture are disclosed in U.S. Pat. Nos.5,266,024 and 6,471,937, both herein incorporated by reference in theirentireties.

One or more partial oxidation zones may be used to effect the partialoxidation of the syngas. Parallel oxidation zones facilitate start-upand turn down operations as the volume of crude syngas changessubstantially. Sequential partial oxidation zones can also be used tofacilitate start-up and turn down operations with a sequential partialoxidation zone either being taken off line or put on line or with theflow of oxygen-containing gas to a partial oxidation zone being changedin response to the change in crude syngas flow. Each partial oxidationzone may have one or more ports for the introduction of each of thecrude syngas and the oxygen-containing gas. A partial oxidation zone maycomprise a static mixer to facilitate mixing of the crude syngas and theoxygen-containing gas. Where the crude syngas being passed to thepartial oxidation zone is at a temperature below that desired, thepartially-oxidized syngas from that or another partial oxidation zonemay be used in indirect heat exchange to increase the temperature of thefeed and cool the partially-oxidized syngas.

Preferably the partial oxidation conditions are selected to adjust thehydrogen to carbon monoxide mole ratio and to decrease the concentrationof carbon dioxide in the syngas. In part, these results are obtained bya reverse water gas shift. In general, partial oxidation conditionswherein higher input hydrocarbon (from the crude syngas and potentiallythe added hydrocarbon fuel) is increased, higher hydrogen to carbonmonoxide ratios are obtained, all else being maintained substantiallyconstant. The mole ratio of hydrogen to carbon monoxide in thepartially-oxidized syngas is often between about 0.4:1 to 1.5:1,preferably between about 0.8:1 and 1.3:1. For many anaerobicfermentation microorganisms, carbon monoxide is a preferred substratefor bioconversion to oxygenated organic compound. Moreover, the partialoxidation provides a net increase in this substrate not only because ofthe conversion of hydrocarbons but also due to the reverse water gasshift. The Component Composition of the partially-oxidized syngasfrequently has the compositions set forth in Table II.

TABLE II Preferred Preferred Component Minimum Maximum Minimum MaximumCarbon Monoxide, 25 65 35 60 mole % Hydrogen, mole % 25 65 35 60 CarbonDioxide, mole % 3 30 5 20 Methane, mole % 0.1 1 0.1 0.75 Acetylene,ppm(mole) 1 100 1 30 Ethylene, ppm(mole) 10 200 10 70 Benzene, ppm(mole)2 100 3 30 Tars, naphthalene, 1 500 1 100 ppm(mole) Hydrogen sulfide, 20300 80 300 ppm(mole) Carbonyl sulfide, 0.1 25 0.5 20 ppm(mole) Ammonia,ppm(mole) 10 10000 10 7500 Nitric oxide, ppm(mole) 0.5 1000 0.5 50Hydrogen cyanide, 0.1 50 2 30 ppm(mole) Other, ppm(mole) 100 10000 2010000 (Excluding nitrogen and water)

As can be seen from Table II, the partial oxidation substantiallyattenuates the concentration of numerous components in the crude syngasthereby facilitating any further desired removal of such components fromthe syngas prior to introduction into the fermentation menstruum.

An aspect of this invention pertains to processes for the partialoxidation of a crude syngas where the rate of supply of theoxygen-containing stream to the partial oxidation step is adjusted toachieve a targeted methane concentration in the partially-oxidizedsyngas. By this invention it has been found that at methaneconcentrations above about 0.75 or 1 mole percent on a ComponentComposition basis, the partial oxidation conditions are not sufficientlysevere to effect the sought reduction of acetylene, ethylene and othercomponents that can adversely affect many microorganisms suitable foranaerobic fermentation of syngas to oxygenated organic compound.However, if the partial oxidation conditions are too severe, i.e., themethane concentration in the partially-oxidized syngas falls below about0.1 mole percent on a Component Composition basis, undue loss of carbonmonoxide and hydrogen tends to occur. In many operations, it is desiredto control the partial oxidation to achieve about 0.1 to 0.4 molepercent methane on a Component Composition basis in thepartially-oxidized syngas. Where a fuel is added to the crude syngasduring partial oxidation, the rate of fuel supply is an additionalvariable that can be used to adjust the temperature of the partialoxidation and the concentration of methane in the partially-oxidizedsyngas. The temperature of the partial oxidation is also affected by therate of supply of oxygen-containing gas.

Syngas Cooling

The partially-oxidized syngas is promptly cooled upon exiting thepartial oxidation operation. The cooling may involve one or more unitoperations. Advantageously due to the high temperature of thepartially-oxidized syngas, heat in the syngas is recovered in a steamboiler to provide steam supply for the biomass conversion process.Preferably, the steam generated is at a pressure of between about 750and 1500, say, 900 to 1100, KPa absolute and the temperature of thepartially-oxidized syngas is reduced to between about 120° C. to 180° C.Alternatively, or in addition, the partially-oxidized syngas may be usedin indirect heat exchange with other process streams such as the crudesyngas passing to the partial oxidation or to heat air used to drybiomass.

The cooling in accordance with the processes of this invention involvesat least one direct heat exchange with water. Usually this direct heatexchange occurs only after the partially-oxidized syngas has been cooledto a temperature below about 180° C., preferably below about 150° C. Thedirect heat exchange may involve passing the syngas through water or acountercurrent contact with a water spray, and the syngas is cooled to atemperature below about 100° C., preferably below about 50° C., andoften at a temperature suitable for introduction into a fermentor or forany optional cleanup operation.

The direct heat exchange also serves to remove a portion of hydrogensulfide, ammonia and at least some of the hydrogen cyanide contained inthe partially-oxidized syngas. The amount of hydrogen cyanide removedcan be enhanced by maintaining the cooling water at a pH in the range ofabout 5.5 to 8, say, about 6 to 6.5. Additionally, reactants forhydrogen cyanide such as acetaldehyde can be contained in the coolingwater. Accordingly, the direct heat exchange with water can also serveto remove hydrogen cyanide to levels suitable such that the cooledsyngas can be introduced into the fermentation operation without furthercleanup treatment.

Optional Cleanup

Subsequent to the cooling of the syngas, in some instances additionalcleanup of the syngas may be desired. The additional cleanup may bedesired because of the sensitivity of the particular microorganisms usedfor the anaerobic fermentation to the residual amounts of one or more ofthe components in the syngas. Where the operation of the partialoxidation is primarily sought to lower the concentration of componentssought to be removed as opposed to reducing them to levels tolerable inthe fermentation, additional cleanup will be required. As stated above,the lowering of the concentration of these components facilitates theremoval of these components in subsequent cleanup operations. Moreimportantly, even with wide variations in concentrations of thesecomponents in the crude syngas, the variations in concentration of thesyngas after partial oxidation are attenuated. Hence the equipmentdesign and control systems need not address the wide variations.

One optional cleanup operation is water scrubbing. Hydrogen cyanide canbe removed by water scrubbing or by scrubbing in the presence of areactant. See, for instance, United States Published Patent ApplicationNo. 20110097701 A1, hereby incorporated by reference in its entirety.The water scrubbing also serves to remove at least a portion ofremaining impurities from the syngas such as ethylene, acetylene,ammonia, hydrogen sulfide and carbonyl sulfide. The scrubbing may beconducted in any convenient manner. Often, the temperature of thescrubbing is in the range of about 4° C. to 50° C., and the scrubbingmay be conducted at subatmospheric, atmospheric or superatmosphericpressure, e.g., frequently at about 105 to 1000 KPa absolute. Waterpressure swing absorption can be used if desired. The pH of thescrubbing solution is usually maintained in the range of about 5.5 to 8,preferably between about 6 to 6.5. Reactants for hydrogen cyanide can beadvantageous in that hydrogen cyanide can be converted to less toxiccompounds. Aldehydes are particularly preferred reactants due to theiravailability. Examples of aldehydes include, but are not limited to,formaldehyde, acetaldehyde, and acrolein(prop-2-enal) with formaldehydebeing most preferred.

Another optional cleanup operation is chemical oxidation with one ormore peroxygenated reactants, preferably permanganate such as sodiumpermanganate and potassium permanganate. The chemical oxidation isparticularly effective in reducing the concentration of compounds thathave ethylenic and acetylenic unsaturation and reducing theconcentration of nitric oxide and sulfur compounds. The chemicaloxidation may be conducted using the peroxygenated reactant in anaqueous solution. Often, the temperature of the chemical oxidation is inthe range of about 4° C. to 50° C., and the chemical oxidation may beconducted at subatmospheric, atmospheric or superatmospheric pressure,e.g., frequently at about 105 to 1000 KPa absolute. The pH of thechemical oxidation solution is usually maintained in the range of about5.5 to 8, preferably between about 6 to 6.5.

Another cleanup operation uses chemical scavengers such as sodiumhydroxide, nitric acid, sodium hypochlorite and the like in an aqueousscrubbing solution to remove one or more components from the syngas. Ifa chemical oxidation is used, this type of cleanup operation is usuallyunnecessary.

Carbon dioxide can be removed from the syngas. In most instances, theconcentration of carbon dioxide in the partially-oxidized syngas issufficiently low that a carbon dioxide removal operation is notnecessary to achieve acceptable fermentation performance. However, ifoff gases from the fermentation are recycled, it is possible thatundesirable carbon dioxide build-up could occur. In such instances, acarbon dioxide removal step could be justified during the cleanup of thesyngas especially where no carbon dioxide is being removed from therecycling off gases. Any suitable carbon dioxide removal process may beused including amine extraction, alkaline salt extractions, waterabsorption, membrane separation, adsorptions/desorption, and physicalabsorption in organic solvents. A preferred process for removal ofcarbon dioxide from gases is by contacting the gas with an aqueoussolution containing oxygenated organic compound. This process isdisclosed in U.S. Patent application No. 2008/0305539, filed Jul. 23,2007, herein incorporated by reference in its entirety. See also, U.S.patent application Ser. No. 12/826,991, filed Jun. 30, 2010 hereinincorporated by reference in its entirety, which discloses contacting agas stream with a mixture of water and a surface active agent underpressure to sorb carbon dioxide and phase separating the gas and liquidstream to provide a gas stream with reduced carbon dioxide concentrationto be used as feed to a reactor.

Another optional cleanup operation comprises contacting the syngas withaqueous fermentation medium containing microorganisms being dischargedfrom the fermentation operation, said contacting being under anaerobicfermentation conditions. Components such as hydrogen cyanide andacetylene that are absorbed by the microorganism but are not released ornot readily released can thus be removed with the microorganisms fromthe syngas prior to being introduced into the fermentation operation.The contacting may be by any suitable manner provided that sufficientresidence time of the gas phase exists for mass transfer of thecomponents sought to be removed to the aqueous phase. Apparatus such asbubble column reactors; jet loop reactors; stirred tank reactors;trickle bed reactors; biofilm reactors; and static mixer reactorsincluding, but not limited to, pipe reactors may find application forthis cleanup operation. Oxygenated organic compound is usually producedand can be recovered from the aqueous phase. A discussion ofpre-reactors is provided in copending U.S. patent application Ser. No.13/243,347, filed on Sep. 23, 2011, hereby incorporated in its entiretyby reference.

Fermentation Gas Feed

The cleaned syngas serves as fresh gas feed to the fermentationoperation. The cleaned syngas may be admixed with other gases, includingbut not limited to, syngas from other sources and recycled off gas fromthe fermentation. The syngas from other sources may include, but is notlimited to, syngas from another biomass gasifier, syngas made from othersources of hydrocarbon such as natural gas, gas generated by reformingor partial oxidation of hydrocarbon-containing materials, and gasgenerated during petroleum and petrochemical processing. Thus, the gasfeed to a fermentor may have the same or a different composition as thecomposition of the cleaned syngas. The composition of the syngasgenerated from biomass may be processed to provide a composition which,when admixed with the gases from the other sources, is suitable for agas feed to the fermentation operation. The Component Composition oftypical cleaned syngas is set forth in Table III.

TABLE III Preferred Preferred Component Minimum Maximum Minimum MaximumCarbon Monoxide, 25 70 40 65 mole % Hydrogen, mole % 30 70 40 65 CarbonDioxide, mole % 1 20 3 15 Methane, mole % 0.1 1 0.1 0.75 Acetylene,ppm(mole) 0.1 10 1 5 Ethylene, ppm(mole) 0.1 50 0.5 10 Benzene,ppm(mole) 0.001 30 0.05 10 Tars, naphthalene, 0.001 10 0.001 5 ppm(mole)Hydrogen sulfide, 0.01 30 0.05 20 ppm(mole) Carbonyl sulfide, 0.01 250.05 15 ppm(mole) Ammonia, ppm(mole) 0.5 1000 1 750 Nitric oxide,ppm(mole) 0.5 100 0.5 50 Hydrogen cyanide, 0.001 2 0.001 0.3 ppm(mole)Other, ppm(mole) 100 10000 20 10000 (Excluding nitrogen and water)

Oxygenated Compound, Microorganisms and Fermentation Conditions

The oxygenated organic compounds produced in the processes of thisinvention will depend upon the microorganism used for the fermentationand the conditions of the fermentation. One or more microorganisms maybe used in the fermentation menstruum to produce the sought oxygenatedorganic compound. Bioconversions of CO and H₂/CO₂ to acetic acid,propanol, butanol, butyric acid, ethanol and other products are wellknown. For example, in a recent book concise description of biochemicalpathways and energetics of such bioconversions have been summarized byDas, A. and L. G. Ljungdahl, Electron Transport System in Acetogens andby Drake, H. L. and K. Kusel, Diverse Physiologic Potential ofAcetogens, appearing respectively as Chapters 14 and 13 of Biochemistryand Physiology of Anaerobic Bacteria, L. G. Ljungdahl eds., Springer(2003). Any suitable microorganisms that have the ability to convert thesyngas components: CO, H₂, CO₂ individually or in combination with eachother or with other components that are typically present in syngas maybe utilized. Suitable microorganisms and/or growth conditions mayinclude those disclosed in U.S. patent application Ser. No. 11/441,392,filed May 25, 2006, entitled “Indirect Or Direct Fermentation of Biomassto Fuel Alcohol,” which discloses a biologically pure culture of themicroorganism Clostridium carboxidivorans having all of the identifyingcharacteristics of ATCC no. BAA-624; U.S. Pat. No. 7,704,723 entitled“Isolation and Characterization of Novel Clostridial Species,” whichdiscloses a biologically pure culture of the microorganism Clostridiumragsdalei having all of the identifying characteristics of ATCC No.BAA-622; both of which are incorporated herein by reference in theirentirety. Clostridium carboxidivorans may be used, for example, toferment syngas to ethanol and/or n-butanol. Clostridium ragsdalei may beused, for example, to ferment syngas to ethanol.

Suitable microorganisms and growth conditions include the anaerobicbacteria Butyribacterium methylotrophicum, having the identifyingcharacteristics of ATCC 33266 which can be adapted to CO and used andthis will enable the production of n-butanol as well as butyric acid astaught in the references: “Evidence for Production of n-Butanol fromCarbon Monoxide by Butyribacterium methylotrophicum,” Journal ofFermentation and Bioengineering, vol. 72, 1991, p. 58-60; “Production ofbutanol and ethanol from synthesis gas via fermentation,” FUEL, vol. 70,May 1991, p. 615-619. Other suitable microorganisms include: ClostridiumLjungdahlii, with strains having the identifying characteristics of ATCC49587 (U.S. Pat. No. 5,173,429) and ATCC 55988 and 55989 (U.S. Pat. No.6,136,577) that will enable the production of ethanol as well as aceticacid; Clostridium autoethanogemum sp. nov., an anaerobic bacterium thatproduces ethanol from carbon monoxide. Jamal Abrini, Henry Naveau,Edomond-Jacques Nyns, Arch Microbiol., 1994, 345-351; Archives ofMicrobiology 1994, 161: 345-351; and Clostridium Coskatii having theidentifying characteristics of ATCC No. PTA-10522 filed as U.S. Ser. No.12/272,320 on Mar. 19, 2010. All of these references are incorporatedherein in their entirety.

Suitable microorganisms for bioconversion of syngas to oxygenatedorganic compound generally live and grow under anaerobic conditions,meaning that dissolved oxygen is essentially absent from thefermentation liquid. Adjuvants to the aqueous menstruum may comprisebuffering agents, trace metals, vitamins, salts etc. Adjustments in themenstruum may induce different conditions at different times such asgrowth and non-growth conditions which will affect the productivity ofthe microorganisms. U.S. Pat. No. 7,704,723, hereby incorporated byreference in its entirety, discloses the conditions and contents ofsuitable aqueous menstruum for bioconversion CO and H₂/CO₂ usinganaerobic microorganisms.

Anaerobic fermentation conditions include a suitable temperature, say,between 25° and 60° C., frequently in the range of about 30° to 40° C.The conditions of fermentation, including the density of microorganisms,aqueous menstruum composition, and syngas residence time, are preferablysufficient to achieve the sought conversion efficiency of hydrogen andcarbon monoxide and will vary depending upon the design of thefermentation reactor and its operation. The pressure may besubatmospheric, atmospheric or super atmospheric, and is usually in therange of from about 90 to 1000 KPa absolute and in some instances higherpressures may be desirable for biofilm fermentation reactors. As mostreactor designs, especially for commercial scale operations, provide fora significant height of aqueous menstruum for the fermentation, thepressure will vary within the fermentation reactor based upon the statichead.

The fermentation conditions are preferably sufficient to effect at leastabout 40 or 50 percent conversion of the carbon monoxide in gas feed.For commercial operations, the fermentation operation preferablyprovides a total molar conversion of hydrogen and carbon monoxide in thenet gas feed in the range of about 85 to 95 percent. Due to the lowsolubilities of carbon monoxide and hydrogen in the aqueous phase,achieving these high conversions may require one or more of usingmultiple fermentation reactors and recycling off gas from a reactor.

The rate of supply of the gas feed under steady state conditions to afermentation reactor is such that the rate of transfer of carbonmonoxide and hydrogen to the liquid phase matches the rate that carbonmonoxide and hydrogen are bioconverted. Hence, the dissolvedconcentration of carbon monoxide and hydrogen in the aqueous phaseremains constant, i.e., does not build-up. The rate at which carbonmonoxide and hydrogen can be consumed will be affected by the nature ofthe microorganism, the concentration of the microorganism in the aqueousmenstruum and the fermentation conditions. As the rate of transfer ofcarbon monoxide and hydrogen to the aqueous menstruum is a parameter foroperation, conditions affecting the rate of transfer such as interfacialsurface area between the gas and liquid phases and driving forces areimportant.

To increase the conversion of carbon monoxide and hydrogen in the freshgas feed to the fermentation, off-gas withdrawn from a fermentationreactor may be recycled or passed to a fermentation reactor that issequential in gas feed flow. Where off-gas is recycled, the portion ofoff-gas recycled is generally selected to avoid an undue build-up of theconcentration of inerts and other gases in the fermentation reactor.

Fermentation Reactors

The fermentation reactors used in this invention may be of any suitabledesign; however, preferably the design and operation provides for a highconversion of carbon monoxide and hydrogen to oxygenated organiccompound. Fermentation reactors include, but are not limited to, bubblecolumn reactors; jet loop reactors; stirred tank reactors; trickle bedreactors; biofilm reactors; and static mixer reactors including, but notlimited to, pipe reactors.

One preferred type of reactor design uses biofilms. Cell retention byformation of biofilms is a very good and often inexpensive way toincrease the density of microorganisms in bioreactors. This requires asolid matrix with large surface area for the microorganisms to colonizeand form a biofilm that contains the metabolizing microorganisms in amatrix of biopolymers that the microorganisms generate. U.S. publishedpatent application No. 20080305539; U.S. published patent applicationNo. 20090035848; and U.S. published patent application No. 20080305540,all hereby incorporated by reference in their entireties, disclosemembrane based bioreactors wherein anaerobic bacteria that have theability to convert syngas to ethanol or other liquids have formedbiofilms on the outer surface of hydrophobic membranes with the syngasfed to the bacterial biofilm through the inner surface of the membrane.Such a bioreactor system has been able to directly convert the primarycomponents of synthesis gas, CO and H2/CO₂ to ethanol and other liquidproducts such as butanol, acetic acid, propanol and butyric acid. Inthese systems the gas flows through a porous region of a hydrophobicmembrane and then reaches a biofilm which is hydrophilic.

United States Published Patent Application 20090215153 A1, herebyincorporated by reference in its entirety, discloses contacting syngascomponents such as CO or a mixture of CO₂ and H₂ with a surface of amembrane that contains a biolayer of microorganisms and permeatingliquid to and from the opposite side of the membrane will provide astable system for producing liquid products such as ethanol, butanol,hexanol, and other chemicals. The membrane has an asymmetricconstruction that provides a porous side, referred to herein as abiolayer that provides pores to promote and control the growth ofmicroorganism colonies therein while also exposing a surface over whichto directly feed the microorganisms with syngas. Simultaneously anotherlayer of the asymmetric membrane having less permeability than thebiolayer, herein referred to as a hydration layer, permeates liquid fromthe opposite side of the asymmetric membrane. The liquid productsproduced in the biolayer on the membrane's gas contact side pass throughthe membrane and into a liquid stream that recovers the desired liquidproducts while also supplying nutrients to the biolayer in the reversedirection of liquid product flow.

In membrane type reactors, generally the syngas flows through the gaschamber or channels of the bioreactor system continuously orintermittently. The gas feed gas pressure is in the range of 110 to 7000KPa absolute, preferably about 150 to 1200 KPa absolute. Thedifferential pressure between the liquid and gas phases is managed in amanner that the membrane integrity is not compromised (e.g., the burststrength of the membrane is not exceeded) and the desired gas-liquidinterface phase is maintained.

Particularly suitable forms of asymmetric membranes are porous membraneswith a tight (i.e., having small pores) thin “skin” on one surface ofthe membrane that provides the hydration layer and a relatively opensupport structure underneath the skin that provides the biolayer anddefines the biopores. The skin will typically comprise a semi-permeablelayer having a thickness of from 0.5 to 10 μm. The skinned asymmetricmembrane can include an “integrally skinned” membrane prepared by usingphase inversion of one polymer or a composite membrane, where a thinlayer of a certain material is formed on top of a porous sublayer of asame or different material.

Several asymmetric ultrafiltration membranes are available fromMillipore Corporation (Bedford, Mass.), including but not limited to theAmicon Membranes and the Ultracel PLC Membranes. The Amicon Membranesare made of polyethersulfone and with a range of a nominal MWCO, forexample a nominal MWCO of 30 kDa for Amicon PM30. The Ultracel PLCMembranes, which are composite membranes made from casting theregenerated cellulose membrane onto a microporous polyethylenesubstrate, are available in the pore size range from 5 kDa (PLCCC) to1000 kDa (PLCXK). Additional examples of asymmetric membranes are theMMM-Asymmetric Super-Micron Membranes and BTS Highly AsymmetricMembranes, both available from Pall Corporation (East Hills, N.Y.). TheMMM-Asymmetric Membranes, available in pore size range from 0.1 to 20.0μm, are made of polysulfone and polyvinylpyrrolidone. The BTS HighlyAsymmetric Membranes, available in pore size range from 0.05 to 0.80 μm,are cast of polysulfone with a “cut off” layer of about 10 μm and atotal thickness of about 120 μm. Hollow fiber membrane modulescontaining asymmetric ultrafiltration membranes are commerciallyavailable from a number of membrane manufacturers. For example, theKrosFlo™. Max Module Model KM5S-800-01N from Spectrum Laboratories(Rancho Dominguez, Calif.) has 22.0 m² membrane surface area ofasymmetric polysulfone hollow fiber membranes with 0.5 mm fiber innerdiameter, a tight skin on the lumen side, and a pore rating of 50 kDa.ROMICON™ polysulfone hollow fiber membranes available from Koch MembraneSystems (Wilmington, Mass.) are also asymmetric with the tight skin onthe lumen side. ROMICON cartridge Model HF-97-43-PM50 is a 6-inch modulecontaining fibers of 1.1 mm inner diameter and 50 kDa nominal MWC at 9.0m² total membrane surface area. Membranes of the various geometries andcompositions described above may be used in arrangements of unitaryarrays or assemblies of varied composition in the systems of thisinvention. Any suitable potting technique can be used to collect andprovide the necessary assembly of individual membrane elements. In suchmembranes the gas and liquid can be brought into direct and intimatecontact at the gas contact surface of the biolayer. Liquid is passed inthe liquid side of the membranes via pumping, stifling, or similar meansto remove the ethanol and other soluble products formed; the productsare recovered via a variety of suitable methods.

Another preferred type of fermentation reactors for commercial scaleoperations are deep, tank reactors in which the microorganisms aresuspended in an aqueous menstruum. Deep, tank reactors have a sufficientdepth of the aqueous menstruum to increase time for mass transfer fromthe gas to aqueous phase and thereby enhance conversion of carbonmonoxide and hydrogen. Most often deep, tank reactors are bubble columnreactors, jet loop reactors and stirred tank reactors. Preferablystirred tank reactors are mechanically-assisted liquid distribution tankreactors, or MLD tank reactors where the stifling is insufficient togenerate small bubbles.

Preferably the gas feed is passed through the deep, tank reactors in theform of small bubbles, sometimes microbubbles, to facilitate masstransfer of carbon monoxide and hydrogen. Microbubbles are bubbleshaving a diameter of 500 microns or less. The deep, tank reactor has aheight of at least about 10, often between about 10 or 15 and 30, metersand an aspect ratio of height to diameter of at least about 0.5:1, say,between about 0.5:1 to 5:1, preferably between about 1:1 to 3:1.

The microbubbles of gas feed introduced into the aqueous menstruum canbe generated by any suitable means including spargers and educers.Preferably they are generated by injection of the gas feed with a motiveliquid. In preferred processes, the gas feed is injected into the deep,tank reactor as a relatively stable gas-in-water dispersion. Theinjectors may be jet mixers/aerators or slot injectors. Slot injectorsare preferred, one form of which is disclosed in U.S. Pat. No.4,162,970. These injectors operate using a motive liquid. The injectors,especially slot injectors, are capable of operating over a wide range ofliquid and gas flow rates and thus are capable of significant turn downin gas transfer capability. The use of injectors can provide bettercontrol over the size of the gas bubbles being introduced into theaqueous menstruum and thus the interfacial area between the gas andliquid phases. Changing bubble size thus modulates the mass transfer ofcarbon monoxide and hydrogen to the aqueous menstruum. Additionally, themodulation enables a microbubble size to be generated that results in apreferred, stable gas-in-water dispersion.

The motive liquid may be any suitable liquid for introduction into thereactor and often is advantageously one or more of aqueous menstruum,liquid derived from aqueous menstruum or make-up liquid to replaceaqueous menstruum withdrawn from product recovery. Preferably the motiveliquid comprises aqueous menstruum. The motive liquid for the injectorspreferably comprises sufficient amount of one or more of oxygenatedorganic compound and other surface active agent to enhance the formationof microbubbles where microbubbles are sought.

In a bubble column reactor, the feed gas is introduced at the bottom ofthe vessel and bubbles through the aqueous menstruum. Bubble columnreactors may contain axial-flow promoting devices such as baffles, downdraft tubes and the like although these devices add to the capital costsof the reactors. Hence, most bubble column reactors do not contain thesedevices. While bubble column reactors are typically the most economicaldesign and can provide high conversion efficiencies, other reactordesigns may find utility in commercially viable bioconversionfacilities. A preferred commercial scale operation using bubble columnsuses sequential bubble columns to the gas feed flow. U.S. patentapplication Ser. No. 13/243,062, filed Sep. 23, 2011, herebyincorporated by reference in its entirety, discloses anaerobicfermentation processes using sequential deep, bubble columns to achievehigh conversion of carbon monoxide and hydrogen contained in the gasfeed without incurring carbon monoxide inhibition. The processescomprise the combination of (i) using at least two deep, bubble columnreactors in flow series; (ii) using certain feed gas compositions; (iii)introducing the feed gas by injection with a motive liquid to producemicrobubbles; and (iv) limiting the degree of conversion of carbonmonoxide in the upstream reactor.

The deep, MLD tank reactors use one or more mechanical stirrers. Themechanical stirring should be sufficient to promote the uniformity ofliquid composition through the reactor and need not, and preferably isnot, used as a generator of a significant fraction of the microbubbles.Usually two or more mechanical stirrers are used at different heightswith higher aspect ratio reactors. The design of mechanical stirrers forstirred tank reactors and their positioning within the reactors for verylarge diameter tanks are well within the skill of a stirred tank reactordesigner. Side paddles or side mounted mixers with impellers arefrequently used. Preferably the design of the mechanical stirrers andthe positioning within the reactor take into consideration energy costsin generating the liquid flow to obtain uniformity of the aqueousmenstruum in the reactor. The deep, MLD tank reactor may contain bafflesor other static flow directing devices. Preferred processes foranaerobic fermentation of syngas to produce oxygenated organic compoundare disclosed in U.S. patent application Ser. No. 13/243,426, filed Sep.23, 2011, hereby incorporated by reference in its entirety.

If a jet loop reactor is used as the deep, tank reactor, one or morevertical tubes may be used in the deep tank reactor. It is not essentialthat these tubes extend the entire height of the aqueous menstruum.Positioning of treated gas injectors can be used to direct the flowaround the loop. Preferably, the gas injectors are positioned at the topof a tube to direct the aqueous menstruum and microbubble dispersiondownwardly. Thus, the initial static head, and the driving force formass transfer of carbon monoxide into the liquid phase, is lower thanthat if the gas feed were introduced at a bottom portion of the deep,tank reactor.

Preferred start-up procedures for deep, tank reactors using injectors tosupply the gas feed are disclosed in U.S. patent application Ser. No.13/243,159, filed Sep. 23, 2011, hereby incorporated by reference in itsentirety. The processes involve increasing both the volume of theaqueous menstruum and the density of the microorganism culture in theaqueous menstruum during start up while modulating the supply of gasfeed to achieve both robust growth of the microorganism culture whileavoiding the risk of carbon monoxide inhibition.

Especially for commercial scale operations, it may be desired topre-react the gas feed to a deep, tank reactor to reduce the risk ofcarbon monoxide inhibition. The pre-reactor may be of any suitableconfiguration including, but not limited to, bubble column reactors,especially bubble column reactors having an aqueous menstruum height ofless than about 10 meters, preferably less than about 5 meters; jet loopreactors; stirred tank reactors; trickle bed reactors; biofilm reactors;and static mixer reactors including, but not limited to, pipe reactors.The pre-reaction often converts between about 10 and 40 percent of thecarbon monoxide in the gas feed to oxygenated organic compound. Apre-reaction operation is disclosed in U.S. patent application Ser. No.13/243,347, filed Sep. 23, 2011, hereby incorporated by reference in itsentirety.

Product Recovery

The fermentation vessel may have added from time to time or continuouslyone or more streams of water, nutrients or adjuvants, andmicroorganisms. A portion of the aqueous menstruum is withdrawn fromtime to time or continuously from the reactor for product recovery.Usually, the withdrawal is made at a point at the upper portion of theaqueous menstruum in the vessel. Product recovery can consist of knownequipment arrangements for removal of residual cell material, separationand recovery of liquid products from the fermentation liquid, return ofrecovered fermentation liquid and purging of waste streams andmaterials. Suitable equipment arrangements can include filters,centrifuges, cyclones, distillation columns, membrane systems and otherseparation equipment. US 2009/0215139 A1 shows an arrangement for aproduct recovery reactor that recovers an ethanol product from abioreactor, herein incorporated by reference in its entirely.

Carbon Dioxide Removal

Carbon dioxide may be removed from at least one of the aqueous menstruumin a reactor or from the off-gas from a reactor where the off-gas isrecycled or passed to a subsequent fermentation reactor. Any suitablecarbon dioxide removal process may be used including amine extraction,alkaline salt extractions, water absorption, membrane separation,adsorptions/desorption, and physical absorption in organic solvents.Considerable flexibility exists in the carbon dioxide removal step inthat certain amounts of carbon dioxide are to be fed to the sequentialreactor. In preferred aspects of the invention, the off-gas after carbondioxide removal will contain at least about 15, say, between 15 and 50,mole percent of total hydrogen and carbon monoxide. Preferably thecarbon dioxide concentration in the off-gas after carbon dioxide removalis between about 2 and 40, more preferably between about 5 or 10 and 20,mole percent. The off-gas after carbon dioxide removal may contain atleast about 5, and often about 10 to 20, mole percent nitrogen.

A preferred process for removal of carbon dioxide from gases is bycontacting the gas with an aqueous solution containing oxygenatedorganic compound. This process for removing carbon dioxide from gas tobe fed to a reactor, including between sequential fermentation stages,is disclosed in U.S. Patent application No. 2008/0305539, filed Jul. 23,2007, herein incorporated by reference in its entirety. See also, U.S.patent application Ser. No. 12/826,991, filed Jun. 30, 2010, hereinincorporated by reference in its entirety, which discloses contacting agas stream with a mixture of water and a surface active agent underpressure to sorb carbon dioxide and phase separating the gas and liquidstream to provide a gas stream with reduced carbon dioxide concentrationto be used a feed to a reactor. United States published patentapplication 2008/0305539 A1 discloses the use of membranes to removecarbon dioxide from a membrane supported fermentation system to preventdilution of concentrations of carbon monoxide and hydrogen in amultistage system.

If desired, a portion of the carbon dioxide dissolved in the liquidphase of the aqueous menstruum can be removed. Any convenient unitoperation for carbon dioxide removal can be used, but the preferredoperation is separation by reducing the pressure to atmospheric or lowerpressure to flash carbon dioxide gas from the liquid phase.

DRAWINGS

A general understanding of the invention and its application may befacilitated by reference to FIG. 1. FIG. 1 is a schematic depiction ofan apparatus generally designated as 100 suitable for practicing theprocesses of this invention. FIG. 1 omits minor equipment such as pumps,compressors, valves, instruments and other devices the placement ofwhich and operation thereof are well known to those practiced inchemical engineering. FIG. 1 also omits ancillary unit operations. Theprocess and operation of FIG. 1 will be described in the context of therecovery and production of ethanol. The process is readily adaptable tomaking other oxygenated products such as acetic acid, butanol, propanoland acetone.

The discussion of the drawings also encompasses a description of acomputer simulation of a process producing 4530 kilograms of ethanol perhour.

Conveyor line 102 provides biomass to the process for conversion toethanol. For purposes of the discussion, the biomass is wood chips.About 20,600 kilograms per hour of wood chips having a moisture contentof about 40 to 50 mass percent are supplied via line 102 to dryer 104.Dryer 104 uses direct heat exchange with hot gases from line 132 to drythe wood chips. The dried wood chips now containing about 15 to 20 masspercent water are conveyed via line 106 to the gasification unit. Thegases used for the drying are exhausted from dryer 104 via line 136.

The gasification unit may be of any suitable design. For purposes ofillustration herein, a gasification unit is an indirect gasificationunit having (i) gasification reactor 108 in which the biomass iscontacted with a recirculating, heat transfer medium (for illustration,sand) and steam, and (ii) combustion reactor 120 in which char producedin the gasification reactor is combusted and sand reheated for recycleto gasification reactor 108. In further detail, the dried wood chips arepassed via line 106 to gasification reactor 108. Gasification reactor108 contains hot sand to provide the heat for the gasification, and thesand may also provide some catalytic activity. Also steam is provided togasification reactor 108 via line 138 for reaction with the biomass.Crude syngas exits gasification reactor 108 at a temperature betweenabout 830° C. and 870° C. and a pressure of about 135 KPa absolute. Charand sand are also withdrawn with the crude syngas. Tars may exist on thechar and the sand. Line 110 directs the effluent from gasificationreactor 108 to cyclone 112.

Cyclone 112 serves to separate the solids from the crude syngas. Thecrude syngas passes from cyclone 112 to partial oxidation reactor 144via line 114. The solids separated in cyclone 112 are passed via line118 to combustion reactor 120. Air is provided via line 122 tocombustion reactor 120. The air may be preheated to facilitate thecombustion. Char is combusted in combustion reactor 120 to reheat thesand to a temperature of between about 1020° C. and 1100° C. Thecombustion gases and reheated sand are passed to cyclone 128 via line126. The reheated sand that is separated by cyclone 128 is passed togasification reactor 116 via line 142. The hot combustion gases and ashfrom cyclone 128 are exhausted via line 130 and passed to cyclone 134.Ash is removed from cyclone 134 via line 140. The hot combustion gasesare exhausted from cyclone 134 through line 132 and may be used forpreheating the combustion air fed to combustion reactor 120 and then fordrying the biomass.

The hot, crude syngas in line 114 contains about 42 mole percenthydrogen, 25 mole percent carbon monoxide and 19 mole percent carbondioxide. (All gas compositions set forth herein are on an anhydrousbasis unless otherwise specified.) Hot heat transfer particles arewithdrawn from gas conditioning reactor 116 via line 132 and passed togasification reactor 108 to provide heat for the gasification.

The hot, crude syngas in line 114 is passed to partial oxidation reactor144. Partial oxidation reactor 144 may be of any suitable design. Asshown in FIG. 1, partial oxidation reactor 144 is also fed a mixture ofnatural gas supplied by line 146 and oxygen supplied by line 147. Themixture is formed in section 144 a of partial oxidation reactor 144under conditions such that essentially all of the natural gas isoxidized prior to the mixture combining with the hot, crude syngas.About 260 kilograms per hour of natural gas are supplied and about 2000kilograms of oxygen are supplied. The temperature of the partialoxidation is between about 1400° C. and 1450° C. Partially-oxidizedsyngas exits partial oxidation reactor 144 via line 148. Thepartially-oxidized syngas contains about 46 mole percent hydrogen, 41mole percent carbon monoxide, 12 mole percent carbon dioxide and 0.15mole percent methane.

The partially oxidized syngas is directed by line 148 to waste heatrecovery boiler 150. Water is provided to waste heat recovery boiler 150by line 152 and steam at a pressure of about 1030 KPa gauge is producedand withdrawn via line 154. This steam can be used as the steam sourcefor gasification reactor 108 and for reboiler heat for ethanoldistillation and for other uses within the process. Thepartially-oxidized syngas is cooled to about 135° C. in waste heatrecovery boiler 150 and is passed via line 156 to first stage scrubber158. The partially-oxidized syngas is at a pressure of about 35 KPagauge.

The first stage scrubber is in essence a quench tower with water beingsprayed to contact an up-flow of syngas. It should be understood thatthe contact time between the liquid and gas phases need only berelatively short in order to effect the sought cooling. If desired,longer contact times can be used to increase removal of water solublecomponents such as hydrogen sulfide, hydrogen cyanide, and the like fromthe syngas.

First stage scrubber 158 is depicted as a quench tower in which waterrecirculated in line 160 from the bottom of first stage scrubber 158 issprayed at the top to contact and cool the partially-oxidized syngas.Line 160 contains a heat exchanger to cool the recirculating water.Condensate from the bottom of first stage scrubber 158 is withdrawn vialine 162 and can be sent to waste water treatment. Thepartially-oxidized syngas is then passed via line 164 to second stagescrubber 166 in which water recirculated in line 170 from the bottom ofsecond stage scrubber is sprayed at the top to contact and cool thepartially-oxidized syngas. Condensate is withdrawn via line 172 fromsecond stage scrubber 166.

The partially-oxidized syngas is cooled to about 35° C. and exits secondstage scrubber via line 174 and is passed to compressor 176. Thefunction of compressor 176 will depend upon whether the syngas isdirectly fed to fermentor 214 or must provide the syngas at a pressuresuitable for additional cleanup operations. As shown, the syngas isdirected to a water pressure swing absorption operation. Accordingly,the pressure of the syngas is increased to about 750 KPa gauge and thecompressed syngas exits via line 178. Due to the increase in pressurecondensate is formed and is removed from the compressed syngas throughline 180.

FIG. 1 depicts the use of both a water pressure swing absorption and apermanganate oxidizer. This is for purposes of illustration. In practiceneither or only one of these optional cleanup operations for the syngaswould typically be used unless very high purity syngas is sought for thefermentation. Line 178 directs the syngas to absorption column 182 ofthe water pressure swing adsorption unit. Water at a temperature ofabout 7° C. is provided to the absorption column via line 184. Thewater, if desired, can contain other components to assist in the removalof components from the syngas such as buffers and reactants such asaldehydes, hypochlorites, peroxygenates, and the like. The spent waterabsorbent exits absorption column via line 186 and is passed todesorption column 188. Desorption column is operated at aboutatmospheric pressure and a temperature of about 7° C. Desorbed gasesexit desorption column 188 via line 192. The rejuvenated water sorbentis withdrawn from desorption column via line 184 for return toabsorption column 182. Make-up water is provided to desorption column188 by line 190. A purge stream is removed from line 184 through line194. Syngas exits absorption column 182 via line 196 and is directed topermanganate oxidizer 198.

Permanganate oxidizer contains a column of water having about 500 ppm(mass) of sodium permanganate dissolved therein and operates at atemperature of about 38° C. and pressure of about 750 KPa gauge. Thewater solution is recirculated via line 200. Manganese dioxide, aco-product of the oxidation, is filtered from the recirculating watersolution and removed via line 204. Make-up sodium permanganate is addedvia line 202.

The treated syngas is withdrawn from permanganate oxidizer 198 via line206 and directed to sacrificial reactor 208. Sacrificial reactor 208 isoptional and frequently serves to mitigate the risk of any toxins to themicroorganisms passing into the fermentation reactor due to an upsetduring the production and cleanup of the syngas. Sacrificial reactor 208contains aqueous fermentation medium containing microorganisms beingpurged from the fermentation operation. Purging of microorganisms isused to retain an advantageous average cell retention time in thefermentation reactors. Sacrificial reactor is maintained underfermentation conditions, e.g., a temperature of about 38° C. and willresult in the bioconversion of syngas to ethanol subject to themicroorganisms not being rendered inactive or killed by a toxin. Aqueousfermentation medium is provided to sacrificial reactor 208 via line 212and aqueous fermentation menstruum is withdrawn via line 213. Ethanol isrecovered from the withdrawn fermentation medium as will be discussedlater.

Syngas is withdrawn from sacrificial reactor 208 via line 210 and isused as the gas feed for anaerobic fermentation to make ethanol. Thefresh feed, for purposes of illustration, contains about 51 mole percenthydrogen, 45 mole percent carbon monoxide, 3 mole percent carbondioxide, about 0.2 mole percent methane, about 5 ppm (mole) acetylene,about 15 ppm (mole) ethylene, about 10 ppm (mole) nitric oxide, and lessthan 1 ppm (mole) hydrogen cyanide (all on an anhydrous basis).

Any suitable anaerobic fermentation process can be employed. Forpurposes of illustration, a fermentation process using sequential deep,bubble column reactors is discussed. It is also possible to use a singlestage fermentation reactor system. As shown, the fresh gas feed ispassed via line 210 to bubble column 214. Bubble column 214 containsaqueous fermentation menstruum at a depth of about 20 meters and ismaintained at a temperature of about 38° C. The gas feed is injected atthe bottom of bubble column 214 using slot injectors and aqueousmenstruum as the motive fluid. The injectors provide microbubbles.Nutrients and make-up water are provided to bubble column 214 via line216. Aqueous menstruum is removed from bubble column 214 via line 218for product recovery and passed to centrifuge 238. The rate of aqueousmenstruum removal is sufficient to maintain an ethanol concentration inbubble column 214 at about 2.5 mass percent. About 60 percent of thehydrogen in the gas feed and 90 percent of the carbon monoxide in thegas feed are consumed in bubble reactor 214. Centrifuge 238 provides asupernatant liquor containing ethanol which is directed to thedistillation operation as will be discussed later. A concentratedmicroorganism-containing stream is produced by centrifuge 238 and aportion is returned to bubble column 214 via line 240 and anotherportion containing the cells intended to be purged, is passed to line212 for transport to sacrificial reactor 208. While only a single bubblecolumn 214 and centrifuge 238 is depicted, it is understood that inpractice multiple bubble columns 214 and centrifuges 238 will be used.Moreover, the centrifuges are typically dedicated to a single reactor toprevent cross contamination.

Off-gas is withdrawn from the top of bubble column 214 via line 220. Theoff-gas contains about 36 mole percent hydrogen, 8 mole percent carbonmonoxide, 54 mole percent carbon dioxide, 0.5 mole percent methane, and1 mole percent nitrogen. The off-gas is passed to carbon dioxide removalunit 222 which is a water pressure swing absorption operation andreduces the carbon dioxide concentration to about 15 to 20 mole percentand is passed to secondary bubble column 226. Secondary bubble column226 contains aqueous fermentation menstruum at a depth of about 20meters and is maintained at a temperature of about 38° C. The gas feedis injected at the bottom of secondary bubble column 226 using slotinjectors and aqueous menstruum as the motive fluid. The injectorsprovide microbubbles. Nutrients and make-up water are provided to bubblecolumn 226 via line 216. Aqueous menstruum is removed from bubble column226 via line 218 for product recovery and passed to centrifuge 238. Therate of aqueous menstruum removal is sufficient to maintain an ethanolconcentration in bubble column 214 at about 2.5 to 3.0 mass percent.Centrifuge 238 provides a supernatant liquor containing ethanol which isdirected to the distillation operation as will be discussed later. Aconcentrated microorganism-containing stream is produced by centrifuge238 and a portion is returned to bubble column 226 via line 240 andanother portion containing the cells intended to be purged, is passed toline 212 for transport to sacrificial reactor 208. While only a singlecentrifuge 238 is depicted, it is understood that in practice multiplecentrifuges 238 will be used. Moreover, the centrifuges are typicallydedicated to a single reactor to prevent cross contamination.

The off-gas from secondary bubble column 226 is passed by line 228 totail gas scrubber 230 for removal of ethanol. Tail gas scrubber 230 usesthe supernatant distillation bottoms from the distillation operationprovided by line 234 to remove the ethanol. The treated gas exits tailgas scrubber via line 232 and can be passed to a thermal oxidation unitto provide heat for drying the wood chips. The spent distillationbottoms in tail gas scrubber 230 is withdrawn via line 236 and returnedto secondary bubble column 226.

Returning to sacrificial reactor 208, the withdrawn aqueous fermentationmedium is passed via line 213 to centrifuge 244. The supernatant liquidis passed from centrifuge 244 via line 246 to line 242 which transportsthe supernatant liquor from centrifuge 238 to the distillationoperation. The solids are removed from centrifuge 244 via line 248 andpassed to waste treatment.

Line 242 carries the combined supernatant liquors to distillation column250 for ethanol recovery. Ethanol (about 92 to 94 weight percent) isrecovered at the top of column 250 and is directed via line 252 toproduct storage or additional operation such as molecular sievetreatment and denaturing for a salable product. About 900 kilograms ofdistillate are produced per hour.

Distillate bottoms are removed from distillation column 250 by line 254which directs them to hydrocyclone 256. The supernatant liquid fromhydrocyclone 256 is passed via line 234 to tail gas scrubber 230. Thesolids are removed from hydrocyclone 256 via line 258.

It is claimed:
 1. A process for continuously converting biomass intooxygenated organic compound comprising: (a) continuously gasifyingbiomass at elevated temperature to provide a crude syngas at atemperature of at least about 450° C., said crude syngas having aComponent Composition containing carbon monoxide, hydrogen, and carbondioxide and at least about 3 mole percent methane; at least about 100ppm (mole) hydrogen sulfide, at least about 600 ppm (mole) benzene, atleast about 1000 ppm (mole) ethylene, at least about 500 ppm (mole)acetylene, and at least about 20 ppm (mole) hydrogen cyanide and ash;(b) removing by phase separation a major portion by mass of the ashwhile the crude syngas is maintained at a temperature of at least about350° C. to provide an ash-reduced crude syngas; (c) continuouslycontacting the ash-reduced crude syngas with oxygen under partialoxidation conditions to perform a partial oxidation at a temperature ofbetween about 800° C. to 1700° C. and for a time sufficient to provide apartially-oxidized syngas having a Component Composition containing lessthan about 1 mole percent methane and at least the same molarconcentration of carbon monoxide as contained in the ash-reduced crudesyngas on a Component Composition basis, said partially-oxidized syngashaving a Component Composition containing at least about 80 ppm (mole)hydrogen sulfide, less than 50 ppm (mole) ethylene, less than 10 ppm(mole) acetylene; (d) continuously cooling the partially-oxidized syngasat least partially by contact with water to cool the partially-oxidizedsyngas to a temperature below about 100° C. and provide a quenchedsyngas containing less than about 80 ppm (mole) hydrogen sulfide on aComponent Composition basis; and (e) removing hydrogen cyanide from thequenched syngas by at least one of sorption and chemical reaction toprovide a fermentation gas feed with a Component Composition having ahydrogen cyanide content of less than about 2 ppm (mole); and (f)continuously supplying the gas feed to a fermentation zone having anaqueous menstruum containing microorganisms suitable for convertingsyngas to oxygenated organic compound, said aqueous menstruum beingmaintained under anaerobic fermentation conditions, to produce saidoxygenated organic compound.
 2. The process of claim 1, wherein thepartially-oxidized syngas contains between about 0.1 and 0.75 molepercent methane on a Component Composition basis.
 3. The process ofclaim 1, wherein steps (d) and (e) are performed simultaneously.
 4. Theprocess of claim 1, wherein the oxygenated organic compound comprisesethanol.
 5. The process of claim 2, wherein the partial oxidation isnon-catalytic.
 6. The process of claim 5, wherein the oxygen is admixedwith fuel prior to contact with the crude syngas.
 7. The process ofclaim 6, wherein the fuel comprises at least one of natural gas, fueloil, and ethanol.
 8. The process of claim 7, wherein the reaction withthe fuel is catalytic.
 9. The process of claim 7, wherein the reactionwith the fuel is non-catalytic.
 10. The process of claim 3, wherein thequench is conducted using water containing at least one aldehyde in anamount sufficient to reduce the concentration of hydrogen cyanide.